Gas sensors are based on different principles of action, which are used predominantly depending on the intended use. An exemplary, special application of certain gas sensors is the measurement of the concentration of inflammable or explosive gases in mixtures with air, e.g., of methane in air. A gas concentration to be detected is measured here in a measuring chamber, into which the air-gas mixture to be measured flows due to diffusion or into which it is delivered by means of a pump, by means of the gas sensor measuring elements proper, i.e., with elements to which electric current is applied, such as pellistors, optoelectronic, or semiconductor elements. In the case of the pellistors, one of the two measuring elements is catalytically prepared, while the second measuring element does not have this catalytic preparation. The behavior of the change in the resistance of the catalytically prepared measuring element compared with the second one, which behavior is characteristic of the gas to be detected, can be evaluated by means of prior-art resistance-measuring bridges.
A gas sensor for combustible gases with electrically heatable measuring elements and corresponding resistance-measuring bridges has become known from DE 15 98 578 C3, wherein, e.g., a very fine-mesh screen or a plate provided with very narrow perforations is provided as an ignition barrier in front of the measuring chamber.
Such ignition barriers or flashback barriers are of utmost significance in the gas sensors being described in order to reliably prevent the gas to be detected, which is being burned in the measuring chamber under controlled conditions, from igniting the gas present in the mixture outside the measuring chamber, i.e., for instance, methane in air.
In the case of the use of infrared optical (optoelectronic) elements, it is desirable to have an effective flashback barrier between the measurement space in the gas sensor and the ambient atmosphere. Corresponding gas sensors with flashback barriers are therefore called explosion-proof